The conventional optical devices have adopted a low pixel number imaging element for reducing the size of the device, and can obtain images with sufficiently wide depth of field only by narrowing the aperture stop of the optical system.
Recently, fining pixels of imaging elements achieved a certain progress, and therefore it has become possible to perform more detailed observation and diagnosis, applying the imaging device having a large number of pixels in an endoscope apparatus. In such endoscope devices, when the aperture stop of the optical system is much reduced in order to ensure the depth of field, the quality of the obtained image quality becomes deteriorated due to the diffraction limit. On the other hand, if the aperture stop cannot be narrowed, the depth of field becomes shortened. Thus, endoscope devices, which have an auto focus function to be able to automatically focus within narrow depth of field, have been proposed. For example, Patent Literature 1 discloses an Auto Focusing endoscope which drives the aperture stop and the lens, using brightness information.